Update the mobile_guide page to the new design and link out to Element X by default.

[pixlwave]

This PR reverts #30104 (restoring #30006) and makes the following additional changes:

• Fix yarn build on Windows.
  • Webpack was failing to pack the gradient image defined in the CSS file.
  • Handling the mobile_guide assets in getAssetOutputPath resolved this.
• Set the default mobile app variant to Element X as requested by product.
  • Anyone who wishes to continue linking to Element Classic will need to set mobile_guide_app_variant to "classic" as detailed in the configuration docs.

Linking to #30056 for visibility.

[pixlwave]

And now the screenshot tests fail on Linux. But not my local Linux where they pass. I now appreciate how nice it is to only have to target one OS 🙈

[t3chguy]

@pixlwave looks like its loading the images wrong

double /mobile_guide/

[pixlwave]

Yeah, I've reverted the use of those requires. That wasn't part of the fix on Windows (just seemed nice to do), so 🤞 this should all be happy now.

[pixlwave]

Nice, looks like it passed this time.

[pixlwave]

Hmm, all the screenshots seem to say Element Pro - that doesn't seem right

My bad, completely missed that last time round.

[pixlwave]

@t3chguy Is there anything more you need from me to get this one merged?